The magnetic random access memory (MRAM) is a hopeful non-volatile memory from the viewpoint of high integration and high speed operation. In the MRAM, a magnetic resistance element is used which shows a magnetic resistance effect such as an AMR (Anisotropic Magneto Resistance) effect, a GMR (Giant Magneto Resistance) effect and a TMR (Tunnel Magneto Resistance) effect.
U.S. Pat. No. 5,640,343 discloses a typical MRAM. In this MRAM, in such a way that an erroneous write (write disturbance) to a half-selection cell does not occur at a time of a write operation, the value of a write current is limited to a predetermined write margin. However, in the magnetic resistance element of each memory cell in the MRAM, a variation exists in its characteristic. As the variation in the characteristic of the magnetic resistance element becomes greater, the write margin becomes smaller. As a result, the possibility of the write disturbance becomes higher.
As a technique for suppressing the erroneous write, a synthetic anti-ferromagnetic layer material (SAF) is proposed. For example, U.S. Pat. No. 6,545,906 discloses the MRAM having the SAF structure. The MRAM having the SAF structure uses the magnetic film of N layers (N is an integer of 2 or more) that are coupled to each other in an anti-ferromagnetic manner. U.S. Pat. No. 6,545,906 discloses the cases of N=2 and N=3.
FIGS. 1 and 2 are conceptual views showing the SAF structure of the magnetic resistance element in the MRAM in the related art. This magnetic resistance element 101 includes a lower electrode layer 111, an anti-ferromagnetic layer 112, a fixed magnetic layer (pin layer) 113, a barrier layer 114, a free magnetic layer (free layer) 115 and an upper electrode layer 116. The free magnetic layer 115 includes a first magnetic film 121 and a second magnetic film 122, which are coupled through a thin non-magnetic film 123 in the anti-ferromagnetic manner (N=2). With this anti-ferromagnetic coupling, as indicated by the arrows in FIGS. 1 and 2, the orientations of the spontaneous magnetizations of the first magnetic film 121 and the second magnetic film 122 are oppositely parallel in a stable state. The orientations of the spontaneous magnetizations of first magnetic film 121 and the second magnetic film 122 in the free magnetic layer (free layer) 115 can be inverted. When one of the spontaneous magnetizations is inverted, the other spontaneous magnetization is also inverted such that the oppositely parallel state is kept.
The first magnetic film 121 in the free magnetic layer 115 is laminated through the barrier layer 114 on the fixed magnetic layer 113. FIG. 1 shows [First State] in which the orientation of the spontaneous magnetization of the first magnetic film 121 and the orientation of the spontaneous magnetization of the fixed magnetic layer 113 are [oppositely Parallel], and FIG. 2 shows [Second State] in which the orientation of the spontaneous magnetization of the first magnetic film 121 and the orientation of the spontaneous magnetization of the fixed magnetic layer 113 are [Parallel]. With the magnetic resistance effect, a resistance value (R+ΔR) of the magnetic resistance element 101 in the first state is greater than a resistance value (R) in the second state. In the MRAM employing the SAF structure, this magnetic resistance element 101 is used as the memory cell, and the change in the resistance value is used, and a data is consequently stored in a non-volatile manner. For example, the first state shown in FIG. 1 is related to a data [1], and the second state shown in FIG. 2 is related to a data [0].
FIG. 3 is a plan view showing the orientation of the spontaneous magnetization in the MRAM of the SAF structure. A write word line 153 and a bit line 155 are formed along an S-direction and a T-direction, which are orthogonal to each other. A write current IWL flows through the write word line 153, and a write current IBL flows through the bit line 155. The memory cell (magnetic resistance element 101) is arranged and sandwiched between the write word line 153 and the bit line 155. Here, [Magnetization Easy Axis Direction] in the free magnetic layer 115 in the magnetic resistance element 101 is assumed to be an X-direction, and [Magnetization Hard Axis Direction] is assumed to be a Y-direction. In the MRAM of the SAF structure, the memory cell is arranged such that the magnetization easy axis direction (the X-direction) has the angle of 45 degrees with respect to the S-direction or T-direction. In the stable state, the spontaneous magnetization (the thick arrow mark) of the first magnetic film 121 and the spontaneous magnetization (the thin arrow mark) of the second magnetic film 122 are oppositely paralleled to each other and oriented to the magnetization easy axis direction.
In the MRAM using the SAF structure of U.S. Pat. No. 6,545,906, the write is performed in accordance with the two methods of [Direct Writing Method] and [Toggle Writing Method].
[Toggle Writing Method] is used when in the first magnetic film 121 and the second magnetic film 122, their spontaneous magnetizations Ms, film thicknesses t and one-axis anisotropic magnetic fields Hk are approximately equal. That is, the product Ms1×t1 of a spontaneous magnetization Ms1 and a film thickness t1 of the first magnetic film 121 and the product Ms2×t2 of a spontaneous magnetization Ms2 and a film thickness t2 of the second magnetic film 122 have the following relation.Ms1×t1≈Ms2×t2 
FIGS. 4A and 4B are timing charts showing the write operation in the MRAM employing the toggle writing method. At first, at a time t1, the write current IWL is supplied to the write word line 153, and at a time t2, the write current IBL is supplied to the bit line 155. In succession, at a time t3, the supply of the write current IWL is stopped, and at a time t4, the supply of the write current IBL is stopped. Such execution of a current control inverts the orientations of the spontaneous magnetizations in the first magnetic film 121 and the second magnetic film 122. In short, according to the toggle writing method, the magnetization state of the free magnetic layer 115 is changed such as a toggle switch between [First State] and [Second State] for each write operation.
As mentioned above, in the MRAM employing a toggle writing method, the state is changed for each write operation. Thus, before the data is written, the data (storage data) stored in the target memory cell is read. Only when the storage data and the write data are different, the write operation is executed.
FIG. 5 is a view showing a threshold property in the MRAM employing the toggle writing method. In FIG. 5, the vertical axis and the horizontal axis indicate the write currents IWL, IBL, respectively. The write currents IWL, IBL corresponding to the regions (toggle operation regions) represented as [toggle] on the drawing are supplied to the write word line 153 and the bit line 155, which correspond to [Selection Cell] to which the data are written. Consequently, the toggling operation is performed on the selection cell. Here, this toggling operation region does not have an X-intercept and a Y-intercept. Thus, only the magnetic field resulting from any of the write currents is applied to [Half-Selection Cell] in which any one of the write word line 153 and the bit line 155 is common to the selection cell. Thus, the toggling operation is not performed on the half-selection cell. In this way, according to the MRAM employing the toggle writing method, the erroneous write is greatly reduced, as compared with the typical MRAM described in U.S. Pat. No. 5,640,343. Also, the value of the write current is not required to be strictly controlled. Hence, the write margin is dramatically improved.
The operation region of the MRAM in the toggle writing method is defined as follows. FIG. 6 is a graph showing a threshold property of the MRAM employing the toggle writing method in detail. The vertical axis and the horizontal axis indicate a word line vertical magnetic field HWL and a bit line vertical magnetic field HBL, which are generated by the write currents IWL, IBL, respectively. FIG. 7 is a graph showing the magnetic resistance characteristic of the free magnetic layer 115 of the MRAM employing the toggle writing method. The horizontal axis indicates a magnetic field HX of the magnetization easy axis (X) direction, and the vertical axis indicates the resistance value.
The magnetic field in the magnetization easy axis (X) direction, which is the minimal magnetic field necessary for the toggling operation, is defined as [Flop Field (Spin Flop Field) HSf]. That is, the magnitude of the flop field HSf is defined as the distance between an original point and a point a in FIG. 6. When the free magnetic layer 115 is constituted by equivalent magnetic films of two layers, the flop field HSf is represented by the following equation by using [One-Axis Anisotropic Magnetic Field HK] and [Anti-Ferromagnetic Coupling Magnetic Field HI]HSf={HK(HK+HI)}1/2 
Also, when the magnetic field at the time of the write operation exceeds a certain value, the spontaneous magnetizations of the respective magnetic fields included in the free magnetic layer 115 are oriented in the perfectly same direction. At this time, the operation becomes unstable. The magnetic field at the limit at which it is not unstable is indicated by a curve c in FIG. 6. The magnetic field in the magnetization easy axis (X) direction, which is the magnetic field of the limit, is defined as [Saturation Field] HSat]. That is, the magnitude of the saturation field HSat is defined as the distance between the original point and a curve c in FIG. 6.
In this way, the upper limit and lower limit of the toggling operation region are determined by the flop field HSf and the saturation field HSat, respectively. The magnetic field Hx in the magnetization easy axis direction applied at the time of the write operation is required to belong to the range between the flop field HSf and the saturation field HSat, as shown in FIG. 7. In the MRAM employing the toggle writing method, a technique that can further enlarge this toggling operation region is desired.
Also, in the MRAM employing the toggle writing method, a technique that can reduce the electric power consumption is desired. This is because the write current resulting from the MRAM employing the toggle writing method tends to be greater than the write current resulting from the typical MRAM. As one example, the flop field HSf for the free magnetic layer 115 constituted by the magnetic films of the two layers shown in FIG. 1 and the write magnetic field for the free magnetic layer 115 of a single layer which does not have the second magnetic film 122 and the magnetic film 123 are compared. In both of the free magnetic layers, the one-axis anisotropic fields HK are assumed to be equal. The write magnetic field to the free magnetic layer 115 of the single layer is about HK. On the other hand, when the anti-ferromagnetic coupling magnetic field HI is given by HI=4HK, the flop field HSf is given from the following equation:Hsf=√{square root over (5)}Hk 
In this way, the MRAM employing the toggle writing method requires the write magnetic field equal to about (a root of 5) times, namely, the write current, as compared with the MRAM having the typical single-layer free magnetic layer. Thus, in the MRAM employing the toggle writing method, a technique that can reduce the write current is desired. For this purpose, the flop field HSf is desired to be small. However, in order to reserve the endurance (the thermal disturbance endurance) against the disturbance in the orientation of the spontaneous magnetization of the free magnetic layer 115, which is caused by the thermal disturbance, the one-axis anisotropic magnetic field HK cannot be excessively reduced.
On the other hand, [Direct Writing Method] is used in case that the first magnetic film 121 and the second magnetic film 122 are different from each other in the spontaneous magnetizations Ms, the film thicknesses t and the one-axis anisotropic magnetic fields HK. Here, in order to simply describe, the product Ms1×t1 of the spontaneous magnetization and film thickness of the first magnetic film 121 and the product Ms2×t2 of the spontaneous magnetization and film thickness of the second magnetic film 122 have the following relation.Ms1×t1>Ms2>×t2 
FIGS. 8A and 8B and FIGS. 9A to 9E are views showing a [1] write operation of the MRAM employing the direct writing method. FIGS. 8A and 8B are the timing charts of the write current IWL and the write current IBL. FIGS. 9A to 9E show the orientations of the spontaneous magnetizations of the first magnetic film 121 and the second magnetic film 122 at the respective times of the timing chart. Here, the [1] write implies the write of [1] to the memory cell (magnetic resistance element 101). Specifically, this indicates that the spontaneous magnetization of the first magnetic film 121 is oriented in the −X direction. A [0] write implies the write of [0] to the memory cell (magnetic resistance element 101). Specifically, this indicates that the spontaneous magnetization of the first magnetic film 121 is oriented in the +X direction.
FIGS. 8A and 8B and FIGS. 9A to 9E show the example in which the [1] write is performed on the magnetic resistance element 101 in the state ([0] state) that [0] has been written, and it is set at the state ([1] state) that [1] ha been written. In an initial state, the spontaneous magnetization of the first magnetic film 121 is assumed to be oriented in the +X direction and the spontaneous magnetization of the second magnetic film 122 is oriented in the −X direction. At first, at a time T1, the write current +IWL is supplied to the write word line 153, and at a time T2, the write current +IBL is supplied to the bit line 155. In succession, at a time T3, the supply of the write current +IWL is stopped, and a time T4, the supply of the write current +IBL is stopped. The execution of such current control inverts the orientations of the spontaneous magnetizations in the first magnetic film 121 and the second magnetic film 122. Then, the spontaneous magnetization of the first magnetic film 121 is oriented in the −X direction and set to the [1] write. It should be noted that even if the same writing sequence is executed, the [1] state is held without any change to the [0] state, if the initial state is in the [1] state.
FIGS. 10A and 10B and FIGS. 11A to 11E are views showing the [0] write operation in the MRAM employing the direct writing method. FIGS. 10A and 10B are the timing charts of the write current IWL and the write current IBL. FIGS. 11A to 11E show the orientations of the spontaneous magnetizations of the first magnetic film 121 and the second magnetic film 122 at the respective timings of the timing charts. Those drawings show an example in which the [0] write is performed on the magnetic resistance element 101 in the [1] state, and it is set to the [0] state. In the initial state, the spontaneous magnetization of the first magnetic film 121 is assumed to be oriented in the −X direction and the spontaneous magnetization of the second magnetic film 122 is oriented in the +X direction. At first, at the time T1, the write current −IWL is supplied to the write word line 153, and at the time T2, the write current −IBL is supplied to the bit line 155. In succession, at the time T3, the supply of the write current −IWL is stopped, and the time T4, the supply of the write current −IBL is stopped. The execution of such current control inverts the orientations of the spontaneous magnetizations of the first magnetic film 121 and the second magnetic film 122. Then, the spontaneous magnetization of the first magnetic film 121 is oriented in the +X direction and set to the [0] state. It should be noted that even if the same writing sequence is executed, the [0] state is held without any change to the [1] state, if the initial state is in the [0] state.
As mentioned above, in the MRAM employing the direct writing method, the write state is determined based on the write operation. For this reason, the reading of the data stored in the target memory cell before the data is written is not required, and all of the memory cells to be written may be written.
FIG. 12 is a graph view showing a threshold property in the MRAM employing the direct writing method. In FIG. 12, the vertical axis and the horizontal axis indicate the write currents IWL and IBL, respectively. The write currents IWL and IBL corresponding to the regions (direct regions) represented as [direct] on the drawing are supplied to the write word line 153 and the bit line 155, which correspond to [Selection Cell] to which the data is written. Consequently, the direct write operation is performed on the selection cell. Here, as shown in FIG. 12, this direct region does not have the X-intercept and the Y-intercept. Thus, only the magnetic field resulting from any of the write currents is applied to [Half-Selection Cell] in which any one of the write word line 153 and the bit line 155 is common to the selection cell. Thus, the direct write operation is not performed to the half-selection cell. In this way, according to the MRAM employing the direct writing method, the erroneous write is greatly reduced, as compared with the typical MRAM. Also, the write is possible in the small word current and bit current, as compared with the toggle writing method.
The operation region of the MRAM employing the direct writing method is defined as follows. FIG. 13 is a graph showing the threshold property of the MRAM employing the direct writing method in detail. The vertical axis and the horizontal axis indicate a word line vertical magnetic field HWL and a bit line vertical magnetic field HBL, which are generated by the write currents IWL, IBL, respectively. FIG. 14 is a graph showing the spontaneous magnetization characteristic of the free magnetic layer 115 of the MRAM employing the direct writing method. The horizontal axis indicates the magnetic field HX of the magnetization easy axis (X) direction, and the vertical axis indicates the value of the synthesized spontaneous magnetization of the free magnetic layer 115 in the X-direction.
The magnetic field in the magnetization easy axis (X) direction, which is the minimal magnetic field necessary for the direct operation, is defined as [Direct Write Magnetic Field Hdir]. That is, the magnitude of the direct write magnetic field Hdir is defined as the distance between an original point and a point b in FIG. 13. When the product of Ms1×t1 of the spontaneous magnetization and film thickness of the first magnetic film 121 and the product of Ms2×t2 of the spontaneous magnetization and film thickness of the second magnetic film 122 are different, the direct write magnetic field Hdir is smaller than the flop field HSf. The curve a and the curve c are similar to FIGS. 6 and 7.
In this way, the upper limit and lower limit of the direct write operation region are determined based on the direct write magnetic field Hdir and the flop field HSf, respectively. The magnetic field Hx in the magnetization easy axis direction applied at the time of the write operation is required to belong to the range between the direct write magnetic field Hdir and the flop field HSf, as shown in FIG. 14. The direct write magnetic field Hdir is smaller than the flop field HSf. In short, in the direct writing method, the write operation can be performed in the small write current, as compared with the toggle writing method, and the smaller electric power consumption can be attained. On the other hand, as can be understood from the drawings, in the MRAM employing the direct writing method, the technique, in which the write operation region is narrow and this operation region can be further enlarged, is desired.
It should be noted that as the technique of the typical MRAM that is not based on the toggle writing method, the followings are known.
Japanese Laid Open Patent Application (JP-P2002-151758A) discloses a ferromagnetic tunnel magnetic resistance element. In a free layer of this ferromagnetic tunnel magnetic resistance element, at least 5 layers or more of ferromagnetic magnetic layers and middle layers are laminated. The magnetizations of the ferromagnetic magnetic layers of the two layers located adjacently through the middle layer are arranged in the anti-ferromagnetic manner.
A magnetic memory disclosed in Japanese Laid Open Patent Application (JP-P2003-298023A) contains two magnetic resistance elements and a common wiring interposed between them. A first magnetic resistance element has a first pin layer and a first free layer. The first pin layer includes ferromagnetic layers of even-numbered layers laminated through non-magnetic layers. The first free layer includes a ferromagnetic layer of a single layer or a plurality of ferromagnetic layers laminated through non-magnetic layers. A second magnetic resistance element has a second pin layer and a second free layer. The second pin layer includes a ferromagnetic layer of a single layer or ferromagnetic layers of three layers or more laminated through non-magnetic layers. The second free layer includes a ferromagnetic layer of a single layer or a plurality of ferromagnetic layers laminated through non-magnetic layer.
Japanese Laid Open Patent Application (JP-P2003-331574A) discloses a writing method of MRAM. This writing method has a (1) step of applying a first magnetic field, which is parallel to a hard axis, to a magnetic resistance effect device that has an easy axis and the hard axis; and a (2) step of applying a second magnetic field, which is weaker than the first magnetic field and is parallel to the hard axis, and a third magnetic field parallel to the easy axis, to the magnetic resistance effect device at the same time.
According to the magnetic resistance effect device disclosed in Japanese Laid Open Patent Application (JP-P 2004-87870A), a pin layer has a function as a magnetic field application member for applying an electrostatic field to the free layer. In order to apply an electrostatic field, the magnitude of a leakage magnetic field from the pin layer is set to be a predetermined value or more.
Japanese Laid Open Patent Application (JP-A-Heisei, 5-266651) discloses a magnetic thin film memory device. This magnetic thin film memory device stores data in accordance with the orientation of the magnetization of a magnetic thin film. This magnetic thin film has a lamination structure. Specifically, in this magnetic thin film, a magnetic layer a whose magnetic coercive force is great and a magnetic layer b whose magnetic coercive force is small are laminated through a non-magnetic layer c, such as a/c/b/c/a/c/b/c - - -.